Backport TunnelSettingsV4 from quantum resistance exploratory branch

[acb-mv]

This PR cherry-picks the TunnelSettings version bump (TunnelSettingsV4) from the quantum resistance branch, adding a quantum resistance setting with three states (automatic, offand on) and making V4 the new version of settings. The purpose of this backport is to unblock any other possible changes to settings and avert merging issues.

---

This change is 

[linear]

IOS-501 Backport TunnelSettingsV4 to main

[buggmagnet]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[PaulCharlton]

Hey @acb-mv ... are you already working on the entire IOS quantum implementation? I was just looking over what needs to be done, and ready/willing/able to work on that. However, I do wish to avoid duplication of effort.

[buggmagnet]

Hi @PaulCharlton, that's very kind of you to propose help on this!

The iOS side of quantum resistance will be very limited (mostly UI facade changes, and saving this to the settings on the device)
Most of the functionality will be built in a Rust crate, reusing parts of the desktop implementation, and exposed via an FFI interface that the PacketTunnel will call.

What kind of solution did you have in mind ?

Reviewable status: complete! all files reviewed, all discussions resolved

[PaulCharlton]

Hi @PaulCharlton, that's very kind of you to propose help on this!

The iOS side of quantum resistance will be very limited (mostly UI facade changes, and saving this to the settings on the device) Most of the functionality will be built in a Rust crate, reusing parts of the desktop implementation, and exposed via an FFI interface that the PacketTunnel will call.

What kind of solution did you have in mind ?

Reviewable status: complete! all files reviewed, all discussions resolved

The yes for the scope UI/save settings locally + the automated testing for those.
As for the Rust Crate — seems right at a high level. Noting, IOS actually provides some callbacks to allow custom override of the TLS handshake in its native TLS stack (if used for obfuscation)

I created issue #5800 for tracking of IOS changes for Quantum Secure, and have a [WIP] merge at #5801